This invention relates generally to measuring viscosity and more particularly to a method and apparatus for measuring the change in viscosity of a fluid as the condition of the fluid deteriorates.
Hydraulic oil such as lubricating oil for an internal combustion engine inevitably experiences changes in its viscosity over time as a function of oil grade, temperature, state of deterioration and other operational parameters. Oil viscosity is an important property because it defines the oil film thickness between moving parts of an engine. It also affects cold crank capability, fuel consumption and for some engines it influences the ability to control emissions such as in a diesel injection system with a hydraulic booster. Oil viscosity may also be used to determine the end of the oil""s useful life. For example, a predetermined threshold value of oil viscosity may be used alone or in conjunction with other oil properties such as oil acidity, particle count, content of certain additives and level of contamination to signify that a volume of oil has reached the end of its useful life and needs to be replaced or reconditioned.
The necessity of changing oil in an internal combustion system such as an automobile is typically determined based on recommendations made by the manufacturer and found in the vehicle""s owner""s manual. Such recommendations are based on assumptions that may or may not apply to a particular user""s specific environmental and/or driving conditions. Changing oil based on the manufacturer""s recommendations may be satisfactory in many circumstances. However, if an inferior grade of oil is used or an engine is operated in a harsh environment the proper interval for oil change may vary as a function of predetermined oil condition parameters, one of which may be viscosity. Consequently, the ability to oil viscosity quickly and accurately may be useful to avoid damage to an engine.
One known method for determining viscosity employs an arrangement that purportedly measures the viscosity of a fluid by determining the time required for a standard element to travel a predetermined distance through the fluid. This arrangement is not capable of in-situ measurements of oil viscosity, which may be desirable in many applications such as in the automobile industry. Other devices and methods are known for measuring viscosity that require samples of the liquid in question to be taken to a laboratory of other facility for analysis. These techniques are not suitable for in-situ measurements.
One exemplary embodiment of the present invention provides an in-situ method and apparatus for measuring the change in viscosity of a lubricant as the quality of the lubricant deteriorates during operational use. One aspect of the present invention allows for the correlation of the heat convection properties of the lubricant to the lubricant""s viscosity. A heating means may be provided for locally increasing the lubricant""s temperature. At least a portion of the heated lubricant may then rise within the bulk lubricant volume due to the heated portion""s reduced density. An operational rise time may then be determined, which may be determined after a vehicle is shut down, for example, to determine the necessity of changing or treating the bulk lubricant. The operational rise time may be the difference between the actuation of a heat pulse from the heating means to heat a portion of the bulk lubricant and the arrival of the heated portion at a predetermined distance from the heating means. The operational rise time may then be used to determine the bulk fluid""s viscosity.
An alternate embodiment allows for the operational rise time to be the amount of time it takes the heated portion to rise from a position proximate the point at which it is heated to a position proximate a second point that is a known distance from the first point. Yet another alternate embodiment allows for determining the heated portion""s average velocity, which may be determined as a function of the temperature difference between the heated lubricant portion and the rest of the bulk lubricant volume. The average velocity may also be a function of shear forces within the lubricant, which are determined by the lubricant""s viscosity.
One aspect of the present invention allows for creating a set of look up tables containing baseline rise times at a known temperature for a set of bulk fluids having a known viscosity. The operational rise time may then be compared to the appropriate baseline rise time contained in the look up table to determine the viscosity change of the bulk lubricant. The baseline rise time data may be interpolated to determine qualitative and/or quantitative information regarding the viscosity of the bulk lubricant in response to determining the operational rise time. This information may then be used to determine whether the bulk lubricant needs to be changed or treated.
One embodiment of the present invention allows for a temperature sensor to take a first temperature at a predetermined height to establish a baseline temperature of the bulk lubricant at that height. The heating means may heat a portion of the bulk lubricant at a predetermined point below the predetermined height. The amount of time it takes the heated portion to rise from the heating means to the predetermined height may be measured by determining when the temperature change occurs at the predetermined height. A change, if any, in the viscosity of the fluid may then be determined.
Another aspect of an exemplary embodiment of the present invention allows for at least one temperature sensor to be used in a quantity of bulk oil for measuring a temperature as a quantity of heated lubricant passes a known point. In an alternate embodiment two temperature sensors may be used to measure temperature change between a first point and a second point in the bulk oil as the heated oil passes between those points.